Allow creating response models

Currently I can't see that response models are supported, its an important part of the API definition especially if a SDK will be created from it.

It seems sensible to create a new attribute to be able to define the responses and their definitions so they can be stored in the gateway for the use case of creating a meaning full SDK / Swagger file.

No file, test, or entry point is named in the issue. First map how Chalice defines API responses and how AWS API Gateway stores them, then determine the attribute shape and the required SDK or Swagger output; done means response models and their definitions are persisted and represented in that output.

Written by the indexing model from the issue text.